Plant Growth and Development

What to remember

• Plant growth is generally indeterminate. It continues throughout life because of meristems. Growth is measured as arithmetic growth (Lt = L0 + rt) or geometric growth (W1 = W0 e^rt), and the overall curve is sigmoid.
• A cell passes through three phases of growth: meristematic, elongation and maturation. Differentiation, dedifferentiation and redifferentiation explain how cells change their role. Development means growth plus differentiation, and plasticity lets one plant form different structures.
• Auxins (apical dominance, rooting, parthenocarpy), gibberellins (stem elongation, bolting), cytokinins (cell division, delay of senescence), ethylene (fruit ripening) and abscisic acid (stress, dormancy, stomatal closure) each have typical effects.
• Auxins, gibberellins and cytokinins are mainly growth promoters. Abscisic acid and ethylene are mainly growth inhibitors.

Growth and its phases

Growth is a permanent and irreversible increase in the size, volume, weight or cell number of an organ or the whole organism. It needs energy and the synthesis of new material. At the cell level, growth is mainly an increase in the protoplasm. This is hard to measure directly, so other parameters are used. These include fresh weight, dry weight, length, area, volume and cell number.

Plant growth is generally indeterminate. Plants keep adding new cells at certain places throughout their life. These places are the meristems. In animals, growth is determinate, because it stops at a certain stage.

Phases of growth. Cells formed at the meristem go through three phases.

Growth rates and conditions for growth

Growth can be shown as a graph of size against time.

Arithmetic growth. After mitosis, only one daughter cell keeps dividing. The other daughter cell differentiates and matures. The growth curve is a straight line. A root that elongates at a constant rate is an example. In words: length at time t = original length + (growth rate × time). In symbols: Lt = L0 + rt. Here, Lt is the length at time t, L0 is the length at the start, and r is the growth rate (the elongation per unit time).

Geometric growth. Both daughter cells keep dividing. At the start the growth is slow (lag phase). It then rises rapidly (log or exponential phase). Finally it slows down because nutrients become limited (stationary phase). The curve is an S-shape, called a sigmoid curve. In words: final size = initial size × e raised to the power of (relative growth rate × time). In symbols: W1 = W0 e^rt. Here, W1 is the final size (weight, height or number), W0 is the initial size at the start, r is the relative growth rate (the efficiency index) and t is time. The letter e is the base of natural logarithms. A population of cells, an organ and a whole plant show the sigmoid curve.

Absolute and relative growth rate. The absolute growth rate is the measurement and comparison of the total growth per unit time. The relative growth rate is the growth of a given system per unit time expressed on a common basis, for example per unit of initial parameter. Take two leaves that each grow by the same amount in a day. The absolute growth rate is the same. But the smaller leaf has the higher relative growth rate, because the same growth is a larger share of its starting size.

Conditions for growth. Growth needs several things.

Differentiation, development and plasticity

Differentiation. Cells formed from the root apical and shoot apical meristems and from the cambium mature to do specific jobs. This change is differentiation. For example, tracheary elements lose their protoplasm and form a strong, lignocellulosic secondary cell wall. This helps them transport water to long distances.

Dedifferentiation. Living cells that are already differentiated and have lost the power to divide can regain it under certain conditions. This is dedifferentiation. Examples are the formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.

Redifferentiation. The meristems and tissues formed by dedifferentiation again lose the power to divide and mature to do specific jobs. This is redifferentiation. The secondary xylem, secondary phloem and cork formed from the cambia are examples.

Development. Development is the sum of all the changes an organism goes through during its life, from seed germination to senescence. In plants, it includes growth and differentiation. It is controlled by intrinsic factors (inside the plant) and by extrinsic factors (outside the plant). Intracellular factors include genes. Intercellular factors include the plant growth regulators. External factors include light and temperature.

Plasticity. Plants follow different pathways in response to the environment or to the phase of life, to form different kinds of structures. This ability is called plasticity. A good example is heterophylly, when the leaves of a plant have different shapes. In cotton, coriander and larkspur, the juvenile leaves are different from the mature leaves in shape. In buttercup, the leaves differ because of the environment. Submerged leaves are narrow and ribbon-like, and aerial leaves are broad.

Plant growth regulators: characteristics and discovery

Plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition. They can be indole compounds (IAA, IBA), adenine derivatives (kinetin), derivatives of carotenoids (ABA), terpenes (GA) or gases (ethylene). They are organic substances produced naturally in plants. They are also called plant hormones when they are made by the plant itself. They control growth and differentiation at very low concentrations. Some are made synthetically too. Auxins, gibberellins and cytokinins are the main growth promoters. Abscisic acid and ethylene are the main growth inhibitors.

Physiological effects of plant growth regulators

Auxins. The name comes from the Greek word auxein, which means to grow. Natural auxins are IAA and IBA. Synthetic ones are NAA and 2,4-D. They are made at the growing apices of stems and roots. Effects:

• They start rooting in stem cuttings, which is widely used for plant propagation.
• They promote flowering in plants such as pineapple.
• They help to prevent the early dropping of leaves and fruits. But they also promote the abscission of older mature leaves and fruits.
• They cause apical dominance. The growing apical bud inhibits the growth of the lateral (axillary) buds. When the shoot tip is removed (decapitation), the lateral buds grow. Gardeners use this to make hedges bushy.
• They cause parthenocarpy, the formation of fruit without fertilisation, for example in tomato.
• 2,4-D is used as a weedicide. It kills dicot weeds but does not harm monocot crops.
• They control xylem differentiation and help cell division.

Gibberellins. There are more than 100 gibberellins, called GA₁, GA₂ and so on. GA₃ (gibberellic acid) is the best known. Effects:

• They increase the length of the axis. For example, they increase the length of grape stalks.
• They improve the size and shape of fruits such as apple, and delay senescence. So the fruits can stay on the tree longer to extend the market period.
• Sprayed on sugarcane, they increase stem length, and so the yield of sugar by a large amount.
• They speed up the malting process in the brewing industry.
• They hasten the maturity period of conifers, which leads to early seed production.
• They promote bolting. This is the sudden elongation of the stem just before flowering in rosette plants such as cabbage.

Cytokinins. They are made where cell division is rapid, such as in root apices, developing shoot buds and young fruits. Effects:

• They promote cell division (cytokinesis).
• They help to produce new leaves, chloroplasts in leaves, lateral shoots and adventitious shoots.
• They overcome apical dominance.
• They promote nutrient mobilisation, which helps in the delay of leaf senescence.

Ethylene. It is a simple gaseous hormone. It is made in large amounts by tissues undergoing senescence and by ripening fruits. Effects:

• It affects horizontal growth of seedlings, the swelling of the axis and the apical hook formation in dicot seedlings.
• It promotes senescence and abscission of leaves and flowers.
• It is highly effective in fruit ripening. It increases the rate of respiration during the ripening of fruits.
• It breaks seed and bud dormancy, and starts germination in peanut seeds and sprouting of potato tubers.
• It promotes rapid elongation of petioles and internodes in deep water rice. This helps the leaves and upper parts to stay above water.
• It promotes root growth and root hair formation.
• It starts flowering and synchronises fruit set in pineapple. It also induces flowering in mango.
• Ethephon is the most widely used source of ethylene. It is an aqueous solution that is absorbed and transported within the plant. It slowly releases ethylene. It hastens fruit ripening in tomato and apple and speeds up abscission in flowers and fruits (thinning of cherry and walnut). It promotes female flowers in cucumbers, which prevents self-pollination.

Abscisic acid (ABA). It is a general growth inhibitor and an inhibitor of plant metabolism. Effects:

• It inhibits seed germination.
• It stimulates the closing of stomata and increases the tolerance of plants to stress. So it is called the stress hormone.
• It plays an important role in seed development, maturation and dormancy. By inducing dormancy, it helps seeds to withstand desiccation and other unfavourable conditions.
• In most cases, it acts against gibberellins.

Plant growth regulators work in two ways. Some are growth promoters, which are involved in activities such as cell division, cell enlargement, flowering, fruiting and seed formation. Others are growth inhibitors, which are involved in plant responses to wounds and to stress of biotic and abiotic origin. They also control dormancy and abscission. Sometimes one regulator may promote one process and inhibit another, so the effect of a regulator depends on the stage and the combination of regulators.

NEET traps

• 1. Darwin worked on canary grass coleoptile. Went worked on oat (Avena sativa) and isolated auxin. Do not swap them.
• 2. Kurosawa studied bakane disease of rice and the fungus Gibberella fujikuroi. This led to gibberellins, not to auxins.
• 3. Kinetin was found by Skoog and Miller from autoclaved herring sperm DNA. It is a cytokinin, not an auxin.
• 4. Cousins found ethylene from ripening oranges and bananas. The hormone of ripening is ethylene, not abscisic acid.
• 5. ABA is also called dormin or abscisin II. It is a stress hormone that closes stomata.
• 6. Arithmetic growth gives a straight line (Lt = L0 + rt). Geometric growth gives a sigmoid curve (W1 = W0 e^rt).
• 7. Apical dominance is caused by auxin. Cytokinins overcome it.
• 8. Dedifferentiation gives back the power to divide (as in cork cambium). Redifferentiation makes the new cells mature again.
• 9. Heterophylly in cotton, coriander and larkspur is linked to the phase of life (juvenile and mature leaves). In buttercup, it is linked to the environment.
• 10. 2,4-D kills dicot weeds and spares monocot crops. Ethephon releases ethylene. Gibberellins cause bolting and increase sugarcane stem length.

One-liners

• 1. Plant growth is generally indeterminate because of meristems.
• 2. Root apical meristem and shoot apical meristem give primary growth.
• 3. Vascular cambium and cork cambium give secondary growth.
• 4. The three phases of cell growth are meristematic, elongation and maturation.
• 5. In arithmetic growth, only one daughter cell divides again.
• 6. In geometric growth, both daughter cells divide again, and the curve is sigmoid.
• 7. The sigmoid curve has a lag phase, a log phase and a stationary phase.
• 8. Tracheary elements are an example of differentiation.
• 9. Interfascicular cambium and cork cambium arise by dedifferentiation.
• 10. Buttercup shows heterophylly because of the environment.
• 11. Went isolated auxin from oat (Avena sativa) tips.
• 12. Skoog and Miller discovered kinetin.
• 13. IAA and IBA are natural auxins. NAA and 2,4-D are synthetic.
• 14. Gibberellins promote bolting. Cytokinins delay senescence.
• 15. Ethephon is a source of ethylene. ABA is the stress hormone.

Practice questions

1. Plant growth is generally

   1. limited to the embryo stage
   2. determinate
   3. indeterminate
   4. restricted to the first year only
   Answer

   C. indeterminate

   Plants keep growing throughout life because of meristems.

2. Which meristem is responsible for the increase in girth of a stem?

   1. Intercalary meristem
   2. Root apical meristem
   3. Shoot apical meristem
   4. Vascular cambium
   Answer

   D. Vascular cambium

   Vascular cambium and cork cambium give secondary growth, which increases girth.

3. Cells with dense cytoplasm, thin cellulosic walls and many plasmodesmata are in the

   1. meristematic phase
   2. elongation phase
   3. maturation phase
   4. stationary phase
   Answer

   A. meristematic phase

   These are features of dividing meristematic cells.

4. The growth curve for arithmetic growth is

   1. a curve that rises and then falls to zero
   2. a straight line
   3. a sigmoid curve
   4. a parabola that falls
   Answer

   B. a straight line

   A constant rate of growth gives a straight line, Lt = L0 + rt.

5. In the equation W1 = W0 e^rt, the letter r stands for

   1. the base of natural logarithms
   2. the relative growth rate
   3. the time of growth
   4. the final size
   Answer

   B. the relative growth rate

   r is the relative growth rate (efficiency index); e is the base of natural logarithms.

6. Heterophylly in which plant is caused by the environment?

   1. Cotton
   2. Coriander
   3. Buttercup
   4. Larkspur
   Answer

   C. Buttercup

   In buttercup, submerged leaves are ribbon-like and aerial leaves are broad.

7. F. W. Went isolated auxin from the tips of seedlings of

   1. rice
   2. canary grass
   3. maize
   4. oat (Avena sativa)
   Answer

   D. oat (Avena sativa)

   Went used oat seedling tips, giving the Avena curvature test.

8. Kurosawa studied the bakane (foolish seedling) disease of rice, which is caused by

   1. Gibberella fujikuroi
   2. Rhizopus
   3. Puccinia
   4. Agrobacterium
   Answer

   A. Gibberella fujikuroi

   The fungus Gibberella fujikuroi made seedlings grow abnormally tall, which led to the gibberellins.

9. Kinetin was obtained by Skoog and Miller from

   1. coconut milk
   2. autoclaved herring sperm DNA
   3. ripe oranges
   4. oat seedling tips
   Answer

   B. autoclaved herring sperm DNA

   Kinetin is a modified form of adenine obtained from autoclaved herring sperm DNA.

10. Cousins found that ripened oranges hasten the ripening of stored bananas by giving off

    1. auxin
    2. gibberellin
    3. abscisic acid
    4. ethylene
    Answer

    D. ethylene

    The volatile substance was ethylene.

11. The inhibitors called dormin and abscisin II were later found to be

    1. the same chemical, abscisic acid
    2. two different gibberellins
    3. kinds of cytokinin
    4. forms of auxin
    Answer

    A. the same chemical, abscisic acid

    Both were found to be ABA.

12. Apical dominance is caused by

    1. abscisic acid
    2. gibberellin
    3. auxin
    4. ethylene
    Answer

    C. auxin

    Auxin from the growing apical bud inhibits the lateral buds.

13. Parthenocarpy in tomato can be induced by

    1. auxin
    2. abscisic acid
    3. cytokinin
    4. ethylene
    Answer

    A. auxin

    Auxins cause the formation of fruit without fertilisation.

14. Which growth regulator, sprayed on sugarcane, increases stem length and the yield of sugar?

    1. Ethylene
    2. Auxin
    3. Abscisic acid
    4. Gibberellin
    Answer

    D. Gibberellin

    Gibberellins increase stem length.

15. Ethephon is used in agriculture because it

    1. kills dicot weeds
    2. releases auxin
    3. releases ethylene
    4. breaks down to abscisic acid
    Answer

    C. releases ethylene

    Ethephon is absorbed and slowly releases ethylene, which hastens fruit ripening.

16. Which plant growth regulator stimulates the closure of stomata?

    1. Cytokinin
    2. Abscisic acid
    3. Gibberellin
    4. Auxin
    Answer

    B. Abscisic acid

    ABA is the stress hormone that closes stomata.

17. Given below are two statements: Statement I: Plant growth is generally indeterminate. Statement II: This is because of the presence of meristems at certain places. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Meristems keep adding new cells throughout the life of the plant.

18. Given below are two statements: Statement I: In arithmetic growth, only one daughter cell keeps dividing. Statement II: The growth curve of arithmetic growth is sigmoid. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Arithmetic growth gives a straight line; the sigmoid curve is for geometric growth.

19. Given below are two statements: Statement I: Redifferentiation is the regaining of the power to divide by differentiated cells. Statement II: Tracheary elements are an example of dedifferentiation. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    B. Both Statement I and Statement II are incorrect

    That is dedifferentiation, and tracheary elements are an example of differentiation.

20. Given below are two statements: Statement I: Heterophylly in cotton is caused by the environment. Statement II: Heterophylly in buttercup is caused by the environment. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    In cotton, coriander and larkspur it is linked to juvenile and mature phases.

21. Given below are two statements: Statement I: Auxins promote flowering in pineapple. Statement II: Ethylene induces flowering in mango. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both statements are correct.

22. Given below are two statements: Statement I: Gibberellins promote bolting in rosette plants such as cabbage. Statement II: Cytokinins promote apical dominance. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Cytokinins overcome apical dominance.

23. Which of the following statements about auxins are correct? 1. They start rooting in stem cuttings. 2. They cause apical dominance. 3. They stimulate closure of stomata.

    1. 1 and 2 only
    2. 2 and 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Answer

    A. 1 and 2 only

    Stomatal closure is an effect of ABA, so statement 3 is wrong.

24. Which of the following statements about ethylene are correct? 1. It delays the senescence of leaves and flowers. 2. It promotes rapid elongation of petioles and internodes in deep water rice. 3. It induces flowering in mango.

    1. 1 and 2 only
    2. 2 and 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Answer

    B. 2 and 3 only

    Ethylene promotes senescence and abscission, so statement 1 is wrong.

25. Which of the following statements about the discovery of plant growth regulators are correct? 1. Darwin worked with the coleoptile of canary grass. 2. Went isolated auxin from tips of oat seedlings. 3. Kurosawa studied rice seedlings infected by Gibberella fujikuroi.

    1. 1 and 2 only
    2. 2 and 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Answer

    D. 1, 2 and 3

    All three statements are correct.

26. Given below are two statements: Statement I: The absolute growth rate is the total growth per unit time. Statement II: Of two leaves that grow by the same amount, the smaller leaf has the higher relative growth rate. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    The same growth is a larger share of the smaller starting size.

27. Assertion (A): Plant growth is indeterminate. Reason (R): Meristems divide throughout the life of a plant.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    Continued division of meristem cells explains indeterminate growth.

28. Assertion (A): Abscisic acid is called the stress hormone. Reason (R): Abscisic acid inhibits seed germination.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    B. Both A and R are true but R is not the correct explanation of A

    Both are true, but inhibition of germination does not explain the stress role.

29. Assertion (A): 2,4-D is used to kill weeds in a wheat field. Reason (R): 2,4-D kills monocot plants but spares dicot plants.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    C. A is true but R is false

    2,4-D kills dicot weeds and does not harm monocot crops, so R is false.

30. Assertion (A): Removal of the shoot tip stops the growth of lateral buds. Reason (R): The shoot tip makes auxin, which inhibits the lateral buds.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    D. A is false but R is true

    Removal of the tip releases the lateral buds, so A is false; R is true.

31. Assertion (A): Spraying gibberellin on sugarcane increases the yield of sugar. Reason (R): Gibberellins increase the length of the stem.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    Longer stems give a larger sugar yield.

32. Assertion (A): Ethylene hastens fruit ripening. Reason (R): Ethylene is a gaseous plant hormone.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    B. Both A and R are true but R is not the correct explanation of A

    Both are true, but being a gas does not explain ripening.

33. Match List I with List II. List I: a. Darwin b. Went c. Kurosawa d. Skoog and Miller List II: i. Bakane disease of rice ii. Tips of Avena sativa iii. Kinetin iv. Canary grass coleoptile

    1. a-ii, b-iv, c-i, d-iii
    2. a-i, b-ii, c-iv, d-iii
    3. a-iv, b-ii, c-i, d-iii
    4. a-iv, b-ii, c-iii, d-i
    Answer

    C. a-iv, b-ii, c-i, d-iii

    Darwin: canary grass; Went: Avena; Kurosawa: bakane disease; Skoog and Miller: kinetin.

34. Match List I with List II. List I: a. Auxin b. Gibberellin c. Cytokinin d. Abscisic acid List II: i. Delay of leaf senescence ii. Closure of stomata iii. Parthenocarpy iv. Bolting

    1. a-iv, b-iii, c-i, d-ii
    2. a-i, b-iv, c-iii, d-ii
    3. a-iii, b-iv, c-ii, d-i
    4. a-iii, b-iv, c-i, d-ii
    Answer

    D. a-iii, b-iv, c-i, d-ii

    Auxin causes parthenocarpy, gibberellin bolting, cytokinin delays senescence and ABA closes stomata.

35. Match List I with List II. List I: a. Root apical meristem and shoot apical meristem b. Vascular cambium c. Intercalary meristem d. Cork cambium List II: i. Lies between xylem and phloem ii. At the base of internodes in grasses iii. Forms cork in the cortex region iv. Tips of root and shoot

    1. a-i, b-iv, c-ii, d-iii
    2. a-iv, b-i, c-iii, d-ii
    3. a-iv, b-i, c-ii, d-iii
    4. a-ii, b-i, c-iv, d-iii
    Answer

    C. a-iv, b-i, c-ii, d-iii

    Each meristem is matched with its position.

36. Match List I with List II. List I: a. Differentiation b. Dedifferentiation c. Redifferentiation d. Heterophylly List II: i. Formation of interfascicular cambium ii. Formation of secondary xylem from cambium iii. Formation of tracheary elements iv. Leaf shape of buttercup

    1. a-i, b-iii, c-ii, d-iv
    2. a-iii, b-ii, c-i, d-iv
    3. a-iii, b-i, c-iv, d-ii
    4. a-iii, b-i, c-ii, d-iv
    Answer

    D. a-iii, b-i, c-ii, d-iv

    Tracheary elements: differentiation; interfascicular cambium: dedifferentiation; secondary xylem: redifferentiation.

37. Match List I with List II. List I: a. Meristematic phase b. Elongation phase c. Maturation phase d. Lag phase of the sigmoid curve List II: i. Increase in vacuoles and cell enlargement ii. Slow growth at the start iii. Dense cytoplasm and thin cellulosic walls iv. Wall thickening and specialisation

    1. a-i, b-iii, c-iv, d-ii
    2. a-iii, b-i, c-iv, d-ii
    3. a-iii, b-iv, c-i, d-ii
    4. a-iii, b-i, c-ii, d-iv
    Answer

    B. a-iii, b-i, c-iv, d-ii

    Each phase is matched with its feature.

38. Match List I with List II. List I: a. Ethephon b. Gibberellic acid (GA₃) c. Kinetin d. 2,4-D List II: i. Gibberellin ii. Cytokinin iii. Weedicide for dicots iv. Source of ethylene

    1. a-i, b-iv, c-ii, d-iii
    2. a-iv, b-ii, c-i, d-iii
    3. a-iv, b-i, c-iii, d-ii
    4. a-iv, b-i, c-ii, d-iii
    Answer

    D. a-iv, b-i, c-ii, d-iii

    Ethephon gives ethylene; GA₃ is a gibberellin; kinetin is a cytokinin; 2,4-D is a weedicide.

39. A gardener cuts the shoot tips of a hedge and the lateral buds grow. What is the reason?

    1. Abscisic acid is made in the buds
    2. Removal of the apical bud ends apical dominance
    3. Ethylene is released from the cut
    4. Cytokinins are lost from the plant
    Answer

    B. Removal of the apical bud ends apical dominance

    The shoot tip makes auxin that inhibits lateral buds; removing it releases them.

40. Tomato flowers are sprayed with an auxin before fertilisation. What is the likely result?

    1. Seedless fruits form
    2. The fruits fail to ripen
    3. The flowers fall at once
    4. Only male flowers form
    Answer

    A. Seedless fruits form

    Auxin induces parthenocarpy.

41. Unripe bananas stored with ripe oranges ripen faster. Which substance causes this?

    1. Cytokinin
    2. Gibberellin
    3. Ethylene
    4. Auxin
    Answer

    C. Ethylene

    Ripe fruit give off ethylene, which hastens ripening.

42. Which growth regulator helps seeds to stay dormant and withstand desiccation?

    1. Gibberellin
    2. Cytokinin
    3. Abscisic acid
    4. Auxin
    Answer

    C. Abscisic acid

    ABA induces dormancy and helps seeds withstand dry conditions.

43. A root grows by arithmetic growth at 2 mm per day. Its length is 10 mm at the start. What will its length be after 5 days?

    1. 25 mm
    2. 15 mm
    3. 12 mm
    4. 20 mm
    Answer

    D. 20 mm

    Lt = L0 + rt = 10 + (2 × 5) = 20 mm.

44. Two leaves have initial areas of 10 cm² and 20 cm². Each grows by 5 cm² in one day. Which statement is correct?

    1. The two leaves have different absolute growth rates
    2. The larger leaf has the higher relative growth rate
    3. Both leaves have the same relative growth rate
    4. The smaller leaf has the higher relative growth rate
    Answer

    D. The smaller leaf has the higher relative growth rate

    The absolute growth rate is the same, but 5 is a larger share of 10 than of 20.

45. The petioles and internodes of deep water rice elongate rapidly and keep the leaves above water. Which regulator is mainly responsible?

    1. 2,4-D
    2. Ethylene
    3. Cytokinin
    4. Abscisic acid
    Answer

    B. Ethylene

    Ethylene promotes rapid elongation of petioles and internodes in deep water rice.